U.S. patent application number 11/480020 was filed with the patent office on 2007-01-04 for high-frequency balun.
Invention is credited to Fumio Asamura, Kenji Kawahata, Katsuaki Sakamoto.
Application Number | 20070001779 11/480020 |
Document ID | / |
Family ID | 37588733 |
Filed Date | 2007-01-04 |
United States Patent
Application |
20070001779 |
Kind Code |
A1 |
Asamura; Fumio ; et
al. |
January 4, 2007 |
High-frequency balun
Abstract
In a balun for mutually converting an unbalanced line for
unbalanced input/output and a balanced line for balanced
input/output, the unbalanced line and the balanced line are
microstrip lines including a signal line arranged on one main
surface of a substrate and a ground conductor arranged on the other
main surface of the substrate. The balun further includes a slot
line formed by a aperture line arranged in the ground conductor in
the other main surface. The microstrip line as the unbalanced line
includes one end portion used as an input/output end and the other
end portion that traverses the slot line, electromagnetically
couples to the slot line, and functions as an electric
short-circuited end. The central portion of the microstrip line as
the balanced line traverses the slot line and electromagnetically
couples to the slot line. Both ends of this microstrip line serves
as the input/output ends.
Inventors: |
Asamura; Fumio; (Saitama,
JP) ; Kawahata; Kenji; (Saitama, JP) ;
Sakamoto; Katsuaki; (Saitama, JP) |
Correspondence
Address: |
Muirhead and Saturnelli, LLC;Suite 1001
200 Friberg Parkway
Westborough
MA
01581
US
|
Family ID: |
37588733 |
Appl. No.: |
11/480020 |
Filed: |
June 30, 2006 |
Current U.S.
Class: |
333/26 |
Current CPC
Class: |
H01P 5/10 20130101 |
Class at
Publication: |
333/026 |
International
Class: |
H01P 5/10 20060101
H01P005/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 1, 2005 |
JP |
2005-194302 |
Claims
1. A balun for mutually converting an unbalanced line for
unbalanced input/output and a balanced line for balanced
input/output, wherein said unbalanced line and said balanced line
are microstrip lines including a signal line arranged on one main
surface of a substrate and a ground conductor arranged on the other
surface of the substrate; comprising: a slot line formed by an
aperture line arranged in said ground conductor in the other main
surface of said substrate; wherein a microstrip line as said
unbalanced line includes a first end portion used as an
input/output end and a second end portion that traverses said slot
line, electromagnetically couples to said slot line, and functions
as an electric short-circuited end; and wherein a center portion of
a microstrip line as said balanced line traverses said slot line
and electromagnetically couples to said slot line and both end
portions of said microstrip line as said balanced line are used as
input/output ends.
2. The balun according to claim 1, wherein said second end portion
of the microstrip line as said unbalanced line traverses said slot
line at one end side of the slot line, and the center portion of
the microstrip line as said balanced line traverses said slot line
at the other end side of the slot line.
3. The balun according to claim 1, wherein the microstrip line as
said unbalanced line traverses the center portion of said slot
line, wherein said balanced line includes first and second
microstrip lines which extend in mutually reverse directions viewed
from said slot line, wherein one end portion of said first
microstrip line traverses said slot line at one end side of the
slot line and functions as an electric short-circuited end, and the
other end portion of said first microstrip line is said
input/output end, and wherein one end portion of said second
microstrip line traverses said slot line at the other end side of
the slot line and functions as an electric short-circuited end, and
the other end portion of said second microstrip line is said
input/output end.
4. The balun according to claim 1, wherein the end portion that
functions as said electric short-circuited end of said microstrip
line projects to provide an electric length of .lamda./4 from a
traversing point with said slot line relative to a wavelength of
.lamda. corresponding to a transmission frequency.
5. The balun according to claim 1, wherein in the end portion that
functions as said electric short-circuited end of said microstrip
line, the signal line and the ground conductor of said microstrip
line are electrically connected by an electrode through
connection.
6. The balun according to claim 3, wherein the end portion that
functions as said electric short-circuited end of said microstrip
line projects to provide an electric length of .lamda./4 from a
traversing point with said slot line relative to a wavelength of
.lamda. corresponding to a transmission frequency.
7. The balun according to claim 3, wherein in the end portion that
functions as said electric short-circuited end of said microstrip
line, the signal line and the ground conductor of said microstrip
line are electrically connected by an electrode through
connection.
8. The balun according to claim 1, wherein both end portion sides
of said slot line function as electric open ends.
9. The balun according to claim 8, wherein the both end portions of
said slot line project to provide an electric length of .lamda./4
from respective traversing points to said microstrip line relative
to a wavelength of .lamda. corresponding to a transmission
frequency.
10. The balun according to claim 8, wherein, viewed from respective
traversing points to said microstrip lines, both end portions of
said slot line are provided with broader hollows than a width of
said slot line at a center portion of said slot line.
11. A balun for mutually converting an unbalanced line for
unbalanced input/output and a balanced line for balanced
input/output, comprising: first and second signal lines which are
arranged on one main surface of a substrate and are adjacent and
parallel; a ground conductor which is arranged in the other main
surface of said substrate so as to be superimposed on one end side
of each of said first and second signal lines; and an electrode
through connection which is arranged at one end of said second
signal line and is electrically connected to said ground conductor;
wherein one end side of said first signal line forms a microstrip
line together with said ground conductor to provide said unbalanced
line, and the other end sides of said first and second signal lines
are regarded as said balanced lines.
12. The balun according to claim 11, wherein the other end sides of
said first and second signal lines extend adjacently in parallel,
and then extends in mutually-apart directions, and a ground
conductor that is superimposed by said first and second signal
lines extending in the mutually-apart directions is arranged on the
other surface of said substrate to provide a microstrip line.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a balun used to mutually
convert a unbalanced transmission line and a balanced transmission
line, and in particular relates to a balun that is suitable for use
in a high-frequency band, such as a microwave band and that attains
a wider bandwidth.
[0003] 2. Description of the Related Art
[0004] A balun is known as a transformer for converting from an
unbalanced transmission line to a balanced transmission line and
vice versa, and is used, for example, in an input/output end of a
repeater in a communication system. Various baluns are known, and
one of those is a high-frequency balun using a microstrip line
(MSL) coupling line, known as an unbalanced high-frequency
transmission line. In recent years, in optical communication
systems or the like, information has been transmitted by using UWB
(Ultra Wide Band) as a frequency band, for example, a frequency
band from 3.1 to 10.6 GHz, and with this situation, a wider
bandwidth is required for a high-frequency balun.
[0005] FIG. 1A shows a conventional high-frequency balun formed by
a microstrip line coupling line, and FIG. 1B shows a
cross-sectional view taken along a line A-A in FIG. 1A. The
high-frequency balun includes unbalanced microstrip line 1 used for
unbalanced input/output, and a pair of microstrip lines 2, 3 used
for balanced input/output. In microstrip lines 1 to 3, a
high-frequency wave component travels or propagates by
electromagnetic fields between signal lines 1a, 2a, 3a arranged in
one main surface of substrate 4 made of a dielectric material and
ground conductor 5 formed over the entirety of the other main
surface of substrate 4.
[0006] Unbalanced microstrip line 1 is formed, for example, by
extending signal line 1a from the left end of substrate 4 in the
horizontal direction in drawings. Balanced microstrip lines 2, 3
are formed, for example, by extending a pair of signal lines 2a, 3a
from the lower end of substrate 4 to be close each other and to be
parallel. Tip portions of signal lines 2a, 3a are bent in
directions that are mutually reversed, and each of signal lines 2a,
3a extends along unbalanced microstrip line 1 (i.e., signal line
1a) in parallel. Each tip end of bent portion 2x, 3x in unbalanced
microstrip lines 2, 3 (i.e., signal lines 2a, 3a) is electrically
connected to ground conductor 5 in the other main surface by
electrode through-connection 6, such as a via-hole and a
through-hole. Then, each bent portion 2x, 3x has an electric length
of .lamda./4 relative to wavelength .lamda. corresponding to
transmission frequency (central frequency) f.sub.0, which is a high
frequency. In this case, the tip end of each of bent portion 2x, 3x
is an electric short-circuit end, and each bending point function
as an electric open end.
[0007] In a balun like this, balanced outputs from amplifier 7 in
mutually opposite-phase, using a ground potential as a reference,
are applied to balanced microstrip lines 2, 3 (i.e., signal lines
2, 3) of the high-frequency balun. Then, the balanced outputs in
mutually opposite-phase travel in balanced microstrip lines 2, 3,
using ground potential 5 as a reference potential. Since tip end of
each of bent portions 2x, 3x is an electric short-circuited ends
and each bending point functions as an electric open end, standing
waves W1, W2 in mutually opposite-phase with electric lengths of
.lamda./4 are generated in both bent portions viewed from each
bending point such that the bending points are maximum voltage
displacement points and the tip end points are minimum voltage
displacement points (i.e., zero voltage points). Incidentally,
amplifier 7 further includes an unbalanced input terminal, a power
source terminal connecting to power source Vcc, and a ground
terminal connecting to a ground potential point.
[0008] Then, since each bent portion 2x, 3x of balanced microstrip
lines 2, 3 are mutually close to unbalanced microstrip line 1, both
are electromagnetically coupled. Therefore, standing wave W of
electric length of .lamda./2, which regards both ends as maximum
voltage displacement points in mutually opposite-phase, is induced
in unbalanced microstrip line 1, while center point P between
bending points of balanced microstrip lines 2, 3 is approximately
regarded as a reference point (i.e., null potential point). With
this arrangement, in unbalanced microstrip line 1, the
high-frequency wave component in unbalanced mode between signal
line 1a and ground conductor 5 travels toward the left end side of
unbalanced microstrip line 1, while the opening end of unbalanced
microstrip line 1 (right end in FIG. 1A, maximum voltage
displacement point) is regarded as a starting point. Then, for
example, coaxial cable 8 is connected to unbalanced microstrip line
1, and the high-frequency wave is transmitted to coaxial cable 8 in
unbalanced mode.
[0009] In this way, in the above high-frequency balun, each of bent
portions 2x, 3x of balanced microstrip lines 2, 3 is set to a
length of .lamda./4 relative to wavelength of .lamda. corresponding
to transmission frequency f.sub.0, and then a standing wave of
.lamda./2 is generated. In other words, bent portions 2x, 3x are
resonant with transmission frequency f.sub.0 corresponding to
standing wave of .lamda./2. Then, bent portions 2x, 3x are
electromagnetically coupled to unbalanced microstrip line 1, and
transmission frequency f.sub.0 in unbalanced mode is obtained.
Specifically, in the high-frequency balun using the microstrip line
coupling line, the balanced mode is converted to the unbalanced
mode and vice versa using resonant phenomenon, and transmission
frequency f.sub.0 is obtained. Therefore, as shown in FIG. 2,
single peak characteristic (curve L) is obtained after conversion,
while transmission frequency characteristic (curve K) having a
linear property is provided before conversion, and there is a
problem in that the band width of transmission frequency f.sub.0 is
narrowed.
[0010] Further, as shown in FIG. 3, when microstrip line 15 is
merely branched in parallel, and one branch microstrip line 15a is
made longer (or shorter) than another branch microstrip line 15b by
.lamda./2 with respect to transmission frequency f.sub.0, the
high-frequency wave component in balanced mode in mutually
opposite-phase can be obtained. However, in this case, since only
transmission frequency f.sub.0 corresponding to wavelength .lamda.
is in opposite-phase, it causes a narrow band characteristic.
SUMMARY OF THE INVENTION
[0011] Accordingly, it is an object of the present invention to
provide a high-frequency balun having a flat propagation
characteristic across a wide band without losing the band width of
the transmission frequency during the mutual-conversion between the
balanced line and the unbalanced line.
[0012] According to the first aspect of the present invention, in a
balun for mutually converting an unbalanced line for unbalanced
input/output and a balanced line for balanced input/output, the
unbalanced line and the balanced line are microstrip lines
including a signal line arranged on one main surface of a substrate
and a ground conductor arranged on the other main surface of the
substrate. The balun further includes a slot line formed by an
aperture line arranged in the ground conductor in the other main
surface of the substrate. In this balun, a microstrip line as the
unbalanced line includes a first end portion used as an
input/output end and a second end portion that traverses or crosses
the slot line, electromagnetically couples to the slot line, and
functions as an electric short-circuited end. A center portion of a
microstrip line as the balanced line traverses or crosses the slot
line and electromagnetically couples to the slot line and both end
sides of this microstrip line are used as input/output ends.
[0013] This high-frequency balun uses electromagnetic coupling by
intersecting the microstrip line and the slot line (SL) through the
substrate. When the tip portion of the slot line and the central
point of the microstrip line are crossed and are
electromagnetically coupled to each other, the high-frequency
component is branched from the central point of the microstrip line
to both end sides of the microstrip line in opposite-phase. For
example, U.S. Pat. No. 6,917,332 discloses electromagnetic coupling
like this.
[0014] According to this arrangement, first, the microstrip line as
the unbalanced line is electromagnetically coupled to the slot
line, and the high-frequency wave component travels from the
microstrip line to the slot line. Then, the slot line is
electromagnetically coupled to the microstrip line as the balanced
line at the central portion of the microstrip line. Therefore, the
high-frequency component branches from the central point of the
microstrip line as the balanced line in opposite-phase and travels
toward both ends of the microstrip line. Therefore, the unbalanced
line can be converted to the balanced line. Needless to say, the
balanced line can be converted to the unbalanced line. According to
this arrangement, since the slot line is used, basically, the
bandwidth of the transmission frequency can be made wider and the
transmission characteristic within the passing band can be made
flat.
[0015] In a balun like this, the second end portion of the
microstrip line as the unbalanced line may traverse the slot line
at one end side of the slot line and the center portion of the
microstrip line as said balanced line may traverse the slot line at
the other end side of the slot line. Also, in this arrangement, the
high-frequency component from the microstrip line as the unbalanced
line is electromagnetically coupled to the slot line, and the
high-frequency component branches from the central point of the
microstrip line as the balanced line to each end of the microstrip
line in opposite-phase. Therefore, the unbalanced line and the
balanced line can be mutually converted.
[0016] Also, the microstrip line as the unbalanced line may
traverse the center portion of the slot line, the balanced line may
include first and second microstrip lines which extend in mutually
reverse directions viewed from the slot line, one end portion of
the first microstrip line may traverse the slot line at one end
side of the slot line and function as an electric short-circuited
end, the other end portion of the first microstrip line may be the
input/output end, one end portion of the second microstrip line may
traverse the slot line at the other end side of the slot line and
function as an electric short-circuited end, and the other end
portion of the second microstrip line may be the input/output end.
Also, in this arrangement, the high-frequency component from the
microstrip line as the unbalanced line is electromagnetically
coupled to the first and second microstrip lines as the balanced
lines at both sides branched from the central portion of the slot
line in-phase. Then, the high-frequency component is branched to
the first and second microstrip lines as the balanced lines in
opposite-phase. Finally, the high-frequency signal is branched from
the central points of the first and second microstrip lines in
opposite-phase, and this arrangement acts as a balun.
[0017] In the present invention, an end portion that functions as
the electric short-circuited end of the microstrip line may project
to provide an electric length of .lamda./4 from a traversing point
(i.e., crossing point) to the slot line relative to a wavelength of
.lamda. corresponding to a transmission frequency. With this
arrangement, the energy conversion efficiency from the microstrip
line to the slot line in the transmission frequency can be
enhanced.
[0018] Alternatively, the end portion that functions as the
electric short-circuited end of the microstrip line may be
constructed by electrically connecting a signal line and a ground
conductor of the microstrip line by an electrode through-connection
such as a via-hole or through-hole. This arrangement provides an
electric short-circuited end for wide frequency bands, there is no
frequency selectivity, and therefore the bandwidth of the
transmission frequency can be made wider.
[0019] In the balun according to the present invention, preferably,
both ends of the slot line function as electric open ends. With
this arrangement, the energy conversion efficiency from the
microstrip line to the slot line can be enhanced.
[0020] The both end portions of the slot line may project to
provide an electric length of .lamda./4 from a traversing point
with the microstrip line relative to a wavelength of .lamda.
corresponding to a transmission frequency. With this arrangement,
the energy conversion efficiency can be enhanced.
[0021] Alternatively, viewed from a traversing point on the
microstrip line, both end portions of the slot line that function
as electric open ends are provided with broader hollows than a
width of the slot line at a central portion of the slot line. With
this arrangement, both ends of the slot line function as electric
open ends for wide frequency bands, there is no frequency
selectivity, and therefore the bandwidth of the transmission
frequency can be made wider.
[0022] According to the second aspect of the present invention, a
balun for mutually converting an unbalanced line for unbalanced
input/output and a balanced line for balanced input/output,
comprising first and second signal lines which are arranged on one
main surface of a substrate and are adjacent and parallel, a ground
conductor which is arranged in the other main surface of the
substrate so as to be superimposed on one end side of each of the
first and second signal lines, and an electrode through-connection
which is arranged at one end of the second signal line and is
electrically connected to the ground conductor, wherein one end
side of the first signal line forms a microstrip line together with
the ground conductor to provide the unbalanced line, and the other
end sides of the first and second signal lines are regarded as the
balanced lines.
[0023] This balun is configured while attention is paid to the
microstrip line and the pair of balanced lines. In this
arrangement, since the first and second signal lines share the
ground conductor, for example, the high-frequency component in
unbalanced mode to be input to one end side of the first signal
line exists just like as the high-frequency source between the
first and second signal lines, and the high-frequency components
in-opposite phase each other are generated by electromagnetic
coupling between the first and second signal lines. Therefore, the
unbalanced line and the balanced line can be mutually
converted.
[0024] In a balun like this, preferably, the other end sides of
said first and second signal lines extend adjacently in parallel,
and then extends in mutual-apart directions, and a ground conductor
that is superimposed by the first and second signal lines extending
in the mutually-apart directions is arranged on the other surface
of the substrate to provide a microstrip line. With this
arrangement, the balanced line formed from the microstrip lines in
opposite-phase each other can be obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1A is a plan view showing a conventional high-frequency
balun;
[0026] FIG. 1B is a cross-sectional view taken along line A-A in
FIG. 1A;
[0027] FIG. 2 is a graph showing a transmission frequency
characteristic of the conventional high-frequency balun;
[0028] FIG. 3 is a plan view showing another example of a
conventional high-frequency balun;
[0029] FIG. 4A is a plan view showing a high-frequency balun
according to the first embodiment of the present invention;
[0030] FIG. 4B is a cross-sectional view taken along line A-A in
FIG. 4A;
[0031] FIG. 5A is a view showing an electric filed direction
distribution in the balun taken along line B-B in FIG. 4A;
[0032] FIG. 5B is a view showing an electric filed direction
distribution in the balun taken along line B-C in FIG. 4A;
[0033] FIG. 6 is a plan view showing an application example of the
balun shown in FIGS. 4A and 4B;
[0034] FIG. 7A is a plan view showing a high-frequency balun
according to the second embodiment of the present invention;
[0035] FIG. 7B is a cross-sectional view taken along line A-A in
FIG. 7A;
[0036] FIG. 7C is a view showing an electric filed direction
distribution in the balun taken along line B-B in FIG. 7A;
[0037] FIG. 8 is a plan view showing a high-frequency balun
according to the third embodiment of the present invention;
[0038] FIG. 9A is a plan view showing a high-frequency balun
according to the fourth embodiment of the present invention;
[0039] FIG. 9B is a cross-sectional view taken along line A-A in
FIG. 9A;
[0040] FIG. 9C is a cross-sectional view taken along line B-B in
FIG. 9A;
[0041] FIGS. 10A and 10B are electric equivalent circuit diagrams
for explaining the operation of the balun shown in FIGS. 9A to 9C;
and
[0042] FIG. 11 is plan view showing an application example of the
balun shown in FIGS. 9A to 9C.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
[0043] FIGS. 4A and 4B show a high-frequency balun according to the
first embodiment of the present invention. In FIGS. 4A and 4B, the
same reference numerals are applied to the same elements as FIGS.
1A and 1B, and no redundant explanations are repeated.
[0044] A balun according to the first embodiment and baluns
according to the second and third embodiments, which will be
described later, are basically configured by using slot line 9 for
converting a balanced line to an unbalanced line and vice versa. A
balun according to the first embodiment includes: substrate 4 made
of a dielectric material or the like; microstrip lines 10, 11 each
having a signal lines formed in one main surface of substrate 4;
ground conductor 5 formed on the whole of the other main surface of
substrate 4; and slot line 9 arranged by forming an opening in
ground conductor 5. Microstrip line 10 is a microstrip line of the
unbalanced line for input/output in unbalanced mode. In this
specification, we call microstrip line 10 an unbalanced microstrip
line. Microstrip line 11 is provided for constituting the balanced
line for input/output in balanced mode. In this specification, we
call microstrip line 11 a balanced microstrip line. Slot line 9 for
conversion is arranged in ground conductor 5 in the other main
surface of substrate 4 as aperture line 9a in the horizontal
direction in drawings, of which both ends (i.e., left and right
directions in FIG. 4A) are closed. In slot line 9, the
high-frequency component travels along aperture line 9 by an
electric field and a magnetic filed generated by this electric
field between ground conductor of both sides of aperture line 9a.
FIGS. 5A and 5B respectively show electric field direction
distributions in the balun taken along lines B-B and B-C in FIG.
4A. In FIG. 4A, symbol "{circle around (.cndot.)}" represents an
electric field upward from the other main surface to one main
surface of substrate 4, and symbol "{circle around (x)}" represents
an electric field downward from one main surface to the other main
surface of substrate 4.
[0045] Unbalanced microstrip line 10 extends from one end side
(e.g., the lower end of substrate 4) to be an input/output end for
the high-frequency components in unbalanced mode and traverses or
crosses one end side of slot line 9 (the left end side of slot line
9 in FIG. 4A). The other end side of unbalanced microstrip line 10
(the upper end side in substrate 4) projects from the traversing
point on slot line 9, with the electric length of approximately
.lamda./4 while the wavelength corresponding to transmission
frequency f.sub.0 is set to .lamda., and therefore functions as an
electric short-circuited end for components of transmission
frequency f.sub.0. Incidentally, the electric short-circuited point
is the traversing point with slot line 9.
[0046] Balanced microstrip line 11 has both ends to be input/output
ends for the high-frequency components in balanced mode. In FIG.
4A, the both ends are an upper end and a lower end of substrate 4,
and balanced microstrip line 11 connects these both ends and
extends linearly, and the central portion (center point) of
microstrip line 11 traverses or crosses the other end side (the
right end side of slot line 9 in FIG. 4A) of slot line 9. The end
at the left side (in FIG. 4A) of slot line 9 projects from the
traversing point of unbalanced microstrip fine 10 by .lamda./4, the
end at the right side of slot line 9 projects from the traversing
point of unbalanced microstrip line 11 by .lamda./4, and both ends
function as electric opening ends for transmission frequency
f.sub.0. Incidentally, the electric open point is the traversing
point with microstrip line 10. In the following explanations, a
lower portion from the traversing point of slot line 9 in
unbalanced microstrip line 11 is regarded as balanced microstrip
line 11x and an upper portion from the traversing point is regarded
as balanced microstrip line 11y.
[0047] In a balun like this, for example, high-frequency wave
component P of transmission frequency f.sub.0 in unbalanced mode by
a coaxial cable is applied to the lower end of unbalanced
microstrip line 10. Then, high-frequency wave component P in
unbalanced mode travels as it is and reaches the traversing point
with slot line 9. Here, when a consideration is given to a case in
that electric field E is upward from the other main surface to one
main surface of substrate 4, that is, from ground conductor 5 to
signal line 10a of unbalanced microstrip line 10, an electric field
that crosses slot line 9 in a direction from the lower side to the
upper side of slot line 9 and a magnetic field orthogonal to the
electric field occur, in particular, at the right side of
unbalanced microstrip line 10, as shown in FIGS. 4A, 5A and 5B.
Therefore, with these electric and magnetic fields, the
high-frequency component from unbalanced microstrip line 10 is
converted to the high-frequency component in balanced mode in slot
line 9. Then, the high-frequency component in balanced mode by slot
line 9 travels on slot line 9 to the right side from the traversing
point (crossing point) with unbalanced microstrip line 10. In this
case, since the tip end portion of unbalanced microstrip line 10
function as an electric short-circuited end with respect to
transmission frequency f.sub.0, the traversing point of slot line 9
becomes a minimum voltage displacement point (i.e., null potential
point) with respect to transmission frequency f.sub.0. Also, since
both end portion sides of slot line 9 project from the respective
traversing points on microstrip lines 10, 11 by .lamda./4 and are
electric open ends, the energy conversion efficiency from the
microstrip line to the slot line is enhanced.
[0048] Then, the high-frequency wave component in balanced mode
that travels in slot line 9 is converted into the high-frequency
wave component in unbalanced mode by electromagnetic coupling to
balanced microstrip line 11 that traverses slot line 9 at the right
side of slot line 9. When electric field E across slot line 9 is
directed from the lower side to the upper side, electric field E
from the other main surface to one main surface of substrate 4 is
generated in balanced microstrip line 11x that extends from the
traversing point of slot line 9 to the lower side. Also, electric
field E from one main surface to the other main surface, which is
the opposite direction to the electric field in balanced microstrip
line 11x, is generated in balanced microstrip line 11y that extends
from the traversing point of slot line 9 to the upper side.
[0049] With this arrangement, high-frequency wave component P
branches from the traversing point of slot line 9 in
opposite-phase, provides a so-called serial opposite-phase branched
structure, and travels in balanced microstrip lines 11x, 11y in the
unbalanced mode. Therefore, at the upper and lower ends, that is,
at output ends of balanced microstrip lines 11x, 11y, it is
possible to obtain the high-frequency wave components in balanced
mode in opposite-phase each other, while the ground potential is
regarded as a reference. However, the high-frequency wave component
propagates in unbalanced mode by the electromagnetic field between
the signal line and ground conductor 5 in each of balanced
microstrip lines 11x, 11y in itself.
[0050] Then, when coaxial cables are respectively connected to both
output ends of balanced microstrip line 11, each coaxial cable can
transmit the high-frequency component in unbalanced mode in
opposite-phase each other, and the high-frequency wave component
can be transmitted in balanced mode as a whole. As shown in FIG. 6,
balanced microstrip lines 11x, 11y are extended on substrate 4 and
are connected to, for example, each input terminal of two-input
amplifier 7, thereby facilitating balanced input easily. The ground
terminal of amplifier 7 can be directly connected to ground
conductor 5. Balanced microstrip lines 11x, 11y have the same line
length, and the input in opposite-phase is maintained. Then, for
example, when output from amplifier 7 is in unbalanced mode, the
output thereof can be introduced through microstrip line 15 and can
be transmitted by a coaxial cable.
[0051] According to this arrangement, by using unbalanced
microstrip line 10, slot line 9, and balanced microstrip line 11,
particularly, by an opposite-phase serial branch from slot line 9
to balanced microstrip line 11, high-frequency components in
opposite phase each other can be obtained while propagations on
microstrip lines are in unbalanced mode in itself, the
high-frequency component in unbalanced mode can be converted into
balanced mode. In this case, the end of slot line 9 projects from
the traversing point of the microstrip line by .lamda./4 and
provides an electric open end while the wavelength corresponding to
transmission frequency f.sub.0 is .lamda.. Therefore, frequency
selectivity occurs in the operation of the balun, however, because
the slot line has a smaller Q value in the resonance
characteristics than the microstrip line, a gentle frequency
propagation characteristic is obtained. In the conventional balun
using microstrip lines, as indicated by curve L in FIG. 2, it is
possible to obtain a frequency propagation characteristic with a
single peak characteristic while transmission frequency (center
frequency) f.sub.0 is regarded as the center. On the other hand,
the balun according to the first embodiment shows a flat frequency
propagation characteristic compared with the conventional
balun.
Second Embodiment
[0052] Next, explanations are given of a balun according to the
second embodiment of the present invention with reference to FIGS.
7A to 7c. In FIGS. 7A to 7c, the same reference numerals are
applied to the same elements as FIGS. 4A and 4B.
[0053] In the balun according to the first embodiment, one balanced
microstrip line 11 is arranged and high-frequency wave components
in opposite-phase each other are obtained from both ends of
balanced microstrip line 11, whereby the high-frequency component
in balanced mode is obtained. However, in the balun according to
the second embodiment, a pair, namely, two balanced microstrip
lines 11x, 11y are arranged, both balanced microstrip lines 11x,
11y are used as a balanced transmission line as a whole, and a
high-frequency component in balanced mode is obtained.
[0054] In the balun according to the second embodiment, the other
end portion of unbalanced microstrip line 10 traverses or crosses
the center portion (center point) of slot line 9 that extends in
the horizontal direction in drawings, the wavelength corresponding
to transmission frequency f.sub.0 is set to .lamda., and the tip of
unbalanced microstrip line 10 projects from this traversing point
by the electric length of .lamda./4. Then, balanced microstrip
lines 11x, 11y respectively traverse both end portions of slot line
9. More specifically, microstrip line 11x at the right side in
drawings extends from the lower end of substrate 4, and traverses
slot line 9, and the tip portion projects from this traversing
point by .lamda./4 in electric length. Similarly, microstrip line
11y at the left side in drawings extends from the upper end of
substrate 4, and traverses slot line 9, and the Up portion projects
from this intersection by the electric length of .lamda./4. In this
case, it is assumed that electric line lengths of balanced
microstrip lines 11x, 11y from the lower end and the upper end to
the point traversing slot line 9 are equal. Both ends of slot line
9 respectively project from the traversing points of corresponding
balanced microstrip lines 11x, 11y by .lamda./4 in electric
length.
[0055] In a balun like this, the high-frequency component applied
to unbalanced microstrip line 10 is branched in phase toward both
ends of slot line 9 from the traversing point of slot line 9, as a
so-called opposite-phase parallel branched structure. In other
words, when electric field E is upward from ground conductor 5 to
signal line 10a of unbalanced microstrip line 10, electric field E
that crosses slot line 9 from the lower side to the upper side of
slot line 9 and a magnetic field that is orthogonal to the electric
field are generated at both right and left sides of unbalanced
microstrip line 10. With these electric and magnetic fields, the
high-frequency component in balanced mode travels from the center
point (i.e., traversing point) of slot line 9 to both ends of slot
line 9 in phase.
[0056] Then, the high-frequency wave component traveling from the
center point of slot line 9 for conversion to both end sides
thereof in balanced mode in phase is converted into unbalanced mode
by electromagnetic coupling with balanced microstrip lines 11x, 11y
that traverse slot line 9 at both end portions of slot line 9,
respectively. For example, in balanced microstrip line 11x at the
right side, upward electric field E from the other main surface to
one main surface of substrate 4 is generated by the electric field
distribution that crosses slot line 9. Also, in balanced microstrip
line 11y at the left side, downward electric field E from one main
surface to the other main surface of substrate 4 is generated.
[0057] Therefore, with mutually-opposed electric fields E and the
magnetic fields due to the electric fields, the high-frequency wave
component travels in each of balanced microstrip lines 11x, 11y in
balanced mode in opposite-phases each other. However, the
propagation mode in itself in each microstrip line 11x, 11y is in
unbalanced mode by the microstrip line. With this arrangement, at
output ends of balanced microstrip lines 11x, 11y, high-frequency
wave components in balanced mode in opposite phase each other,
using the ground potential as a reference, can be obtained.
Third Embodiment
[0058] A balun according to the third embodiment of the present
invention shown in FIG. 8 is similar to that of the first
embodiment, however, the balun according to the third embodiment is
different from that of the first embodiment in an arrangement for
setting the tip portion of unbalanced microstrip line 10 to an
electrical short-circuited end and an arrangement for setting both
ends of slot line 9 to electric open ends.
[0059] In the third embodiment, the tip end of unbalanced
microstrip line 10 projects from the traversing point of slot line
9 is connected to ground conductor 5 by via-hole 6 that is arranged
adjacently to the traversing point. Also, both ends of slot line 9
that project from the traversing points of unbalanced microstrip
line 10 and balanced microstrip line 11 are formed so as to be
wider than the width of slot line 9 in the portion between these
two traversing points, that is, the width of aperture line 9a in
ground conductor 5. In this embodiment, both ends of slot line 9
are hollows 9z as circular openings arranged in ground conductor
5.
[0060] According to this arrangement, since both end portions of
slot line 9 are respectively in expanded circular shapes, both ends
of slot line 9 function as electric open ends not only for the
frequency based on the electric length (.lamda./4), as an aperture
line, but also for a wideband of frequencies. Also, since the tip
end of unbalanced microstrip line 10 is connected to ground
conductor 5 by via-hole, that is electrode through connection 6,
the tip end functions as an electric short-circuited end not only
for the frequency based on the line length, but also for a wideband
of frequencies.
[0061] Therefore, the balun of the third embodiment provides no
frequency selectivity, that is, no resonance characteristic in
comparison with the balun according to the first embodiment in
which the electric open end and the electric short-circuited end
are made by using the one-fourth wavelength line. This balun
according to the third embodiment provides the flat frequency
propagation characteristic and is more suitable to use in a wider
band.
[0062] In the balun of the second embodiment, an electric
short-circuited end can be configured by the via-hole arranged in
the tip portion of the microstrip line, and an electric open ends
can be configured by hollows formed at end portions of the slot
line. Incidentally, when no wide band characteristic is required in
particular, the arrangements of the first and second embodiments
can be manufactured easily than the arrangement of the third
embodiment, because no via-hole is required.
Fourth Embodiment
[0063] A balun according to the fourth embodiment of the present
invention shown in FIGS. 9A to 9C is provided with substrate 4 made
of a dielectric material, first and second signal lines 12, 13
arranged in one main surface of substrate 4, and ground conductor 5
arranged in the other main surface of substrate 4. The center area
of ground conductor 5 is opening 14, and the other main surface of
substrate 4 is exposed in opening 14.
[0064] First and second signal lines 12, 13 extend in the
horizontal direction in drawings and traverse or cross opening 14
of the other main surface across substrate 4. All areas of both end
portions of first and second signal lines 12, 13 are superimposed
on ground conductor 5 across substrate 4. Here, first signal line
12 extends from the left end to the right end of substrate 4 and
second signal line 13 extends to the right end of substrate from
the front position of opening 14 apart from the left end of
substrate 44.
[0065] First and second signal lines 12, 13 are arranged in
parallel to be mutually close above opening 14 and are arranged to
be mutually apart in a V-shape from the right end of opening 14
toward the right end of substrate 4. With this arrangement, first
signal line 12 provides a microstrip line in the area except the
position of opening 14, that is, both end areas that are
superimposed on ground conductor 5, and second signal line 13
provides a microstrip line in the area from the right end of the
opening to the right end of the substrate. The left end of second
signal line 13 is electrically connected to ground conductor 5 in
the other main surface of substrate 4 by via-hole 6.
[0066] According to this arrangement, for example, at the left end
side of substrate 4, a core (center conductor) of the coaxial cable
is connected to first signal line 12 and a braided line (outer
conductor) is connected to ground conductor 5, thereby applying the
high-frequency component in unbalanced mode of transmission
frequency f.sub.0 to the balun. The high-frequency component in
unbalanced mode travels toward the left side of opening 14 by the
microstrip line formed by first signal line 12 and ground conductor
5 while unbalanced mode is kept. Then, the high-frequency wave
component in unbalanced mode by the microstrip line does not travel
any more, because no ground conductor 5 exists within opening
14.
[0067] Here, second signal line 13 is connected to ground conductor
6 by via-hole 6 at the left end of second signal line 13 and is
arranged in parallel with first signal line 12 above opening 14.
Because ground conductor 5 is common to first and second signal
lines 12, 13 at the left end of opening 14, as indicated by an
electrical equivalent circuit in FIG. 1A, high-frequency wave
component P that travels in first signal line 12 as the microstrip
line is electromagnetically coupled to second signal line 13 and
functions as high-frequency source e that are connected to both
signal lines 12, 13 in appearance. In this case, while the left end
of second signal line 13 is connected to ground conductor 5 by
via-hole 6, second signal line 13 has inductance L for a
high-frequency component, because of a strip line (thin line).
Also, since first signal line 12 and second signal line 13 are
adjacently arranged in parallel, line-to-line capacitance C is
generated. Therefore, the transmission line by first signal line 12
and second signal line 13 provides a distributed constant circuit,
as shown in FIG. 10B.
[0068] For this reason, the potential of second signal line 13 does
not become the ground potential by ground conductor 5 with respect
to the high-frequency component. Accordingly, high-frequency
component P is transmitted on first signal line 12. However, the
resistance in the distributed constant circuit is basically 0 with
respect to a direct current component, the potential of the first
signal line becomes the ground potential. Charges that have
electrically opposite signs one other by electrostatic coupling
(i.e., capacitive coupling) are generated between first signal line
12 and second signal line 13, and electromagnetic fields having
mutually opposite directions are generated between first signal
line 12 and second signal line 13. Therefore, high-frequency
components in opposite-phase each other travel in first signal line
12 and second signal line 13 from high-frequency source e.
[0069] First signal line 12 and second signal line 13 extend in
directions that are mutually apart, in the right end from opening
14 and are arranged to be superimposed on ground conductor 5 of the
other main surface of substrate 4. Here, while electromagnetic
coupling between first signal line 12 and second signal line 13 is
gradually released, first signal line 12 and second signal line 13
provide each microstrip line together with ground conductor 5 of
the other main surface. The high-frequency component that is
transmitted in opposite-phase each other between first and second
signal lines 12, 13 transmits through each microstrip line between
first and second signal lines 12, 13 as the balanced mode that
maintains the mutually-opposite-phase relationship. The
high-frequency wave component that transmits through each
microstrip line is in unbalanced in itself.
[0070] According to this arrangement, with electromagnetic coupling
between first signal line 12 and second signal line 13 above
opening 14, unbalanced mode of the microstrip line by first signal
line 12 is converted into balanced mode, and this functions as a
balun. In this case, since this conversion does not use the
resonance phenomenon like the conventional balun, it is possible to
obtain the balun that provides a relatively flat frequency
propagation characteristic and can be used in a wide band.
[0071] In the balun according to the fourth embodiment, first
signal line 12 and second signal line 13 provide microstrip lines
together with the ground conductor 5 of the other main surface of
substrate 4 in the area from the right end portion of opening 14 to
the right end of substrate 4. Therefore, similarly to the first
embodiment, for example, as shown in FIG. 11, balanced input to
two-input amplifier 7 arranged on substrate 4 can be carried out
easily. In other words, though amplifier 7 is provided with a power
source terminal and a ground terminal in addition to input/output
terminals, the ground terminal can be connected to ground conductor
5 by a via-hole or the like, and therefore, balanced input of
high-frequency components can be carried out easily. On the other
hand, when no ground conductor is arranged in the other main
surface of substrate 4, it becomes difficult to connect a ground
terminal of amplifier 7 to a ground potential point.
[0072] Incidentally, even if no ground conductor is arranged in the
other main surface of substrate 4 and only first and second signal
lines 12, 13 are arranged in one main surface of substrate 4, the
mutual-conversion function from unbalanced mode to balanced mode
and vice vera is attained. Therefore, a coaxial cable is connected
to the microstrip line by first signal line 12 on the left side of
substrate 4 and balanced cables are connected to first and second
signal lines 12, 13 on the other side, thereby functioning as a
two-way high-frequency balun.
* * * * *